The invention relates generally to fiber optic cable routing systems and more specifically to assemblies for routing optical fiber without violating the minimum bend radius for the fiber.
Optical communications refer to the medium and the technology associated with the transmission of information as light pulses. Many applications utilize an optical fiber network to establish optical communications between network locations. In order to enable optical communication and the flow of optical signals between network locations, various interconnections must be established between different optical fibers.
Optical fiber cable consists of a plurality of optical fibers surrounded by protective sheath. Each individual optical fiber (xe2x80x9cfiberxe2x80x9d) consists of a small diameter core of low-loss material such as glass or plastic surrounded by a protective cladding that has a slightly lower index of refraction than the core. Light, as it passes from a medium of higher index of refraction to one of lower index of refraction, is bent away from the normal to the interface between the two media. At a critical angle of incidence, transmitted light is totally reflected within the medium having the higher index of refraction. Building on these basic rules of physics, optical fibers are designed and made such that there is essentially total reflection of light as it propagates through an optical fiber core. Thus, the core is able to guide light pulses with small attenuation of transmitted light pulses and low signal loss.
In many cases of signal transmission via optical media, a key transmission parameter is signal loss per distance transmitted. Due to the sensitive nature of the core of an optical fiber, there is a need to protect an optical fiber from external sources of stress, such as bending, pressure and strain, that increase signal loss. For example, an optical fiber should not be bent sharply anywhere along its path. If an optical fiber is bent past a critical angle, portions of transmitted light pulses will not be reflected within the core of the optical fiber and will no longer traverse the optical fiber. These attenuated portions of light pulses result in signal loss and thus, degradation of signal quality. Moreover, excess stress on an optical fiber may result in breakage of the fiber resulting in a total signal loss.
Referring to FIG. 1a, there is shown a simple ray model of light pulse transmission on a straight optical fiber. The optical fiber 100, shown in longitudinal cross section, has an optical core 102 which is surrounded by a cladding 104 and has a critical angle xcex8c. FIG. 1b shows a simple ray model of light pulse transmission on a bent optical fiber. As illustrated, when the bend of the optical fiber 100 is such as to cause a light ray to strike the boundary of the core 102 and cladding 104 at an angle greater than the critical angle xcex8cxe2x80x94the angular excess, as shown in the inset, being labeled xcex8bendxe2x80x94the light ray leaks out of the optical fiber core. Further, while lower order mode light rays are not likely to leak out of the optical fiber core, they may be transformed into higher order mode light rays and may leak out at a subsequent bend in the optical fiber. Accordingly, it is necessary that an optical fiber be routed so that bends in the optical fiber are of a sufficient radius to substantially avoid occurrence of such extra critical angle, and the associated light leakage.
The minimum bend radius characterizes the radius below which an optical fiber should not be bent to avoid light ray leakage. Typically, the minimum bend radius varies with fiber design. Bending an optical fiber with a radius smaller than the minimum bend radius may result in increased signal attenuation and/or a broken optical fiber.
Ordinarily, a unique optical fiber routing will be required to transmit light pulses between network locations. Over this unique route, light pulses may be propagated across several different optical fibers. At each transition from one fiber to another, individual optical fibers may be joined together by a splice connection or a connector adapter, thereby enabling light pulses to be carried from/between a first fiber and a second fiber. Once made, a connection must be held securely in place to prevent a loss of transmission quality. Transmission via optical fiber also requires repeating (i.e., amplifying) the transmitted optical signal at distance intervals. Consequently, optical fiber connections also must be made at the distance intervals where such signal repeater equipment is needed.
It may be necessary to bend optical fibers around comers and other obstacles in order to route the optical fiber to/from optical fiber network equipment and accomplish the required connections. While performing such activity, stresses on the optical fiber must be limited. Moreover, connections of optical fibers need to be isolated and protected from environmental degradation, strain, and torque in order to maintain the proper alignment between connected optical fibers and to avoid undesirable signal attenuation.
Previously, closures have been designed to protect connections of copper wire. A closure typically houses a cable interconnect frame and provides mounting surfaces for electronics and apertures for cabling to pass to/from the enclosed space of the closure. A door is usually provided for access to the enclosed interconnect frame and electronics compartment. However, optical fiber closures present a host of different complexities revolving around bend limiting and the minimum bend radius as described above.
Existing closure architecture does not always integrate placed optical fibers well with regard to the maintenance of the minimum bend radius for the fibers. Further, such closures provide no or limited means to control, organize, or stow optical fibers and fiber slack routed inside the closure. For instance, fibers may not physically fit well into a closure or may be jostled upon opening or closing of the closure. Additionally, excess lengths of fiber may not be compatible with the typically constrained storage area available within a closure. Consequently, special care is required in order to open and close the closure without exceeding the bend limit of or breaking optical fibers contained therein, which action could cause a system malfunction. Thus, a technician faces many logistical problems during maintenance of optical fibers within such a closure, adding significant complexity and time to that required to remove, repair and/or replace fibers in a closure.
Accordingly, it is an objective of the invention to provide a space efficient fiber interconnection closure for managing and protecting optical fibers that are terminated and/or spliced at various locations in an optical fiber network. It is also an objective of the invention to provide a closure that maintains the bend limit for fibers routed to termination points within the closure during its opening and closing. It is an additional objective of the invention to provide a slack storage area for optical fibers terminated in the closure. It is a further objective of the invention to provide an area within the closure for storage of excess lengths of optical fiber so as to enable the removal and re-entry of splice organizers from the closure during system maintenance. As a still further objective, the closure also should provide easy access to fiber connections and fiber slack while making accommodations to protect against excessive bending of optical fibers.
To that end, a modular fiber optic interconnection closure is described that maintains bend limits for optical fibers spliced and/or terminated in a protected space. A combination closure according to the invention is comprised of three articulated segmentsxe2x80x94a base assembly, a chassis, and a cover plate, which couple to form an enclosed space in which fibers are routed and connected. The closure arrangement is established to ensure that routed fibers are maintained at bend radii greater than their inherent minimum bend radius, thereby limiting stress on the fibers as well as avoiding light leakage due to excessive bend angles.
Mounting positions for splice organizers, which nominally house optical fiber splices, are provided in the base assembly. Mounting positions for optical adapters/ connectors (xe2x80x9ctermination adaptersxe2x80x9d), which nominally terminate optical fibers, are provided on two-plane panel in the chassis. Fibers are routed to/from these mounted fiber connection devices through a series of fiber-slack storage-devices. Fiber clips, fiber rings and fiber retainers are arranged to retain and organize fibers directed to/from the fiber connection devices and exit portals of the closure. The fiber clips, rings and retainers are positioned in the closure interior to assure that fiber bends are no smaller than a predetermined minimum bend radius while gathering, organizing and strain-relieving fibers. A vertical raceway along the exterior of the closure directs routed fibers to other optical network equipment. The closure maintains bend limits when opened to allow full access to the splices and termination adapters, thereby enabling quick and convenient fiber installation and service. Fiber slack may be manipulated and thus an individual fiber""s connections may be accessed, while secured inside the closure, without disturbing other fiber connections. The particular splice organizers and/or termination adapters mounted in the closure may be specified to suit individual applications. The closure of the invention allows fiber connections to be fashioned and protected from environmental conditions and fiber stress.